The present invention relates to a current/voltage non-linear resistor having main component of zinc oxide (ZnO), applied in an overvoltage protection device such as an arrester or a surge absorber, and in particular, relates to a current/voltage non-linear resistor capable of improving a resistance distribution in the current/voltage non-linear resistor and a component composition of an auxiliary component included in the main component. The present invention also relates to a sintered body for the current/voltage non-linear resistor of the character mentioned above.
In general, overvoltage protection devices such as arresters or surge absorbers are employed in power systems or circuits of electronic equipments to protect the power system or electronic equipments by removing the overvoltage state that is superimposed on the normal voltage. As overvoltage protection devices, current/voltage non-linear resistors are frequently used. The current/voltage non-linear resistors have a characteristic that practically shows an insulating characteristic at an ordinary voltage, but shows low resistance when the overvoltage is applied.
A current/voltage non-linear resistor may be manufactured by procedures described in Japanese Patent Publication No. HEI 4-25681, for example. First of all, a raw material is prepared by adding Bi2O3, Co2O3, MnO, Sb2O3 and NiO as auxiliary component to zinc oxide (ZnO) as main component. This raw material is then thoroughly mixed with water and binder and then granulated by using a spray drier etc, and a sintered body is obtained through molding and sintering processes. Thereafter, an insulating layer is formed on the side surfaces of the sintered body by applying an insulating substance to prevent surface flashover to the side surfaces of the sintered body, followed by a thermal (heat) treatment. After the formation of the insulating layer, the current/voltage non-linear resistor is manufactured by polishing both end surfaces of the sintered body and then attaching electrodes thereto.
However, in recent years, with increased demand for power, increased sub-station capacity and installation of sub-stations underground, a reduction in the size of sub-station equipment has been required.
Although the current/voltage non-linear resistor whose main component is zinc oxide is employed in the arrester on account of its excellent non-linear resistance characteristic, this non-linear resistance characteristic only offers the protection level of the arrester and it is hence necessary to further improve such characteristic.
For example, Japanese Patent Publication No. HEI 4-25681 discloses an attempt to improve the non-linear resistance characteristic and life characteristic by restricting the contents of auxiliary components such as Bi2O3, Co2O3, MnO, Sb2O3 and NiO added to the ZnO as main component.
Furthermore, Japanese Patent Publication No. HEI 2-23008 discloses an attempt to improve life characteristic by restricting the contents of the auxiliary component such as Bi2O3, Co2O3, MnO, Sb2O3 and NiO and restricting the crystal phases of the Bi2O3 contained in the sintered body having the main component of ZnO.
Furthermore, Japanese Patent Laid-open Publication No. HEI 8-264305 discloses an attempt to improve the energy endurance by making the resistance in a peripheral region lower than the resistance in a central region in a sintered body.
However, the characteristics that are required for the conventional current/voltage non-linear resistors are currently becoming increasingly strict, and it becomes difficult to satisfy the characteristics required with the prior arts described above.
Specifically, it becomes also difficult to achieve sufficient equipment reliability and stability of the power supply since a sufficient life characteristic is not obtainable because the normal voltage that is applied to the current/voltage non-linear resistor may be deteriorated.
Furthermore, it is difficult to achieve miniaturization of the arrester since the number of sheets of current/voltage non-linear resistor laminated in the lightning arrester cannot be reduced since the resistance per sheet of the current/voltage non-linear resistor is insufficient.
It is also difficult to minimize transformers and switches for the reason that, although it is required to improve the energy endurance, i.e. the surge, that can be absorbed without damage by the current/voltage non-linear resistor, if the number of sheets of the current/voltage non-linear resistor be reduced, the surge energy endurance obtained would be insufficient.
In view of these problems, an object of the present invention is to provide a voltage/current non-linear resistor in which an excellent current/voltage non-linear resistor resistance characteristic is obtained and which has an excellent life characteristic and energy endurance characteristic.
Another object of the present invention is to also provide a sintered body for the current/voltage non-linear resistor of the characters mentioned above.
In order to achieve these and other objects, the present inventors of the subject application made repeated studies of various types of the component composition of current/voltage non-linear resistors and the resistance distribution, as a result of which the inventors have perfected the present invention.
That is, according to the present invention, there is provided in one aspect a current/voltage non-linear resistor comprising a sintered body having a main component of ZnO, an electrode applied to a surface of the sintered body and an insulation material also applied to the surface of the sintered body, the main component containing, as auxiliary components, Bi, Co, Mn, Sb, Ni and Al, the contents of the auxiliary components being respectively expressed as Bi2O3, Co2O3, MnO, Sb2O3, NiO and Al3+, of Bi2O3: 0.3 to 2 mol %, Co2O3: 0.3 to 1.5 mol %, MnO: 0.4 to 6 mol %, Sb2O3: 0.8 to 7 mol %, NiO: 0.5 to 5 mol % and Al3+: 0.001 to 0.02 mol %; Bi2O3 crystalline phase in the sintered body including an xcex1-Bi2O3 phase representing at least 80% of the total Bi2O3 phase.
The reason why the component composition range and crystalline phase are restricted in this way according to the present invention of the above aspect is that if these ranges are departed from, the non-linear resistance characteristic is adversely affected.
The Bi2O3 that is added as the auxiliary component is a component, existing at the grain boundaries of the ZnO produces a non-linear resistance characteristic. The Co2O3 and NiO are component which, dissolved in a solid solution in the ZnO grains, are effective for improving the non-linear resistance characteristic. Sb2O3 is a component which controls grain growth of the ZnO grains during the sintering process by forming spinel grains and has the action of improving uniformity, conferring the benefit of improving the non-linear resistance characteristic. MnO is a component that is effective for improving the non-linear resistance characteristic by dissolving in the solid solution in the ZnO grains and spinel grains. Al3+ is a component that is effective for improving the non-linear resistance characteristic by dissolving in the solid solution in the ZnO grains, thus lowering the electrical resistance of the ZnO grains.
Furthermore, by restricting the amount of xcex1-Bi2O3 phase in the orthorhombic system to at least 80% of the total bismuth phase, the insulation resistance of the Bi2O3 crystalline phase in the sintered body is raised and the non-linear resistance characteristic can be improved.
In another aspect of the present invention, there is also provided a current/voltage non-linear resistor comprising a sintered body having a main component of ZnO, an electrode applied to a surface of the sintered body and an insulation material also applied to a surface of the sintered body, the main component containing, as auxiliary components, Bi, Co, Mn, Sb, Ni, Al and Te, the contents of the auxiliary components being respectively expressed as Bi2O3, Co2O3, MnO, Sb2O3, NiO, Al3+ and TeO2 of Bi2O3: 0.3 to 2 mol %, Co2O3: 0.3 to 1.5 mol %, MnO: 0.4 to 6 mol %, Sb2O3: 0.8 to 7 mol %, NiO: 0.5 to 5 mol %, Al3+: 0.001 to 0.02 mol % and TeO2: 0.01 to 1 mol %; a Bi2O3 crystalline phase in the sintered body including an xcex1-Bi2O3 phase representing no more than 10% of the total Bi2O3 phase.
According to the present invention of the aspect mentioned above, by making the Te, expressed as TeO2, a content of 0.01 to 1 mol % and by making the ratio represented by xcex1-Bi2O3 phase in the total Bi2O3 phase not more than 10% in the Bi2O3 crystalline phase in the sintered body, the insulation resistance of the Bi2O3 crystalline phase in the sintered body can be made higher and the non-linear resistance characteristic improved. This is because, if the Te content, expressed as TeO2, is made less than 0.01 mol %, the benefit in terms of improvement of insulation resistance of the Bi2O3 crystalline phase is lower, and on the other hand, if the content is made more than 1 mol %, the insulation resistance is lowered. Furthermore, it is because, if the ratio represented by xcex1-Bi2O3 phase in the Bi2O3 crystalline phase in the sintered body is more than 10% of the total Bi2O3 phase, the insulation resistance of the Bi2O3 crystalline phase in the sintered body cannot be made high.
In preferred examples of the above aspects, the sintered body contains 0.005 to 0.05 wt % of Ag expressed as Ag2O. The sintered body contains 0.005 to 0.05 wt % of B expressed as B2O3. The sintered body contains Si of an amount of 0.01 to 1 mol %, expressed as SiO2.
A ratio of the content of the Bi2O3 of the sintered body with respect to the Sb2O3 is less than 0.4.
The sintered body contains Zr in the amount of 0.1 to 1000 ppm, expressed as ZrO2. The sintered body contains Y of an amount of 0.1 to 1000 ppm, expressed as Y2O3. The sintered body also contains Fe of an amount of 0.1 to 1000 ppm, expressed as Fe2O3.
According to these preferred examples, the life characteristic of the current/voltage non-linear resistor can be greatly improved by adding 0.005 to 0.05 wt % of Ag and B, respectively, independently or simultaneously. In the case of the basic composition mentioned above, it is possible for the life characteristic to be insufficient if the charging ratio (the voltage that is normally applied to the current/voltage non-linear resistor) is set to a high level. Accordingly, by adding Ag and B to this basic composition, the change of the leakage current with time is reduced and the life characteristic is improved. The reason for restricting the added content of Ag and B expressed respectively as Ag2O or B2O3 to 0.005 to 0.05 wt % is that, if the added content is less than 0.005 wt %, the benefit of an improvement in the life characteristic is not obtained while, contrariwise, if it is made more than 0.05 wt %, the life characteristic actually deteriorates.
Furthermore, according to the present invention, by restricting the silicon to 0.01 to 1 mol % expressed as SiO2, pores in the sintered body can be reduced and the strength of the sintered body increased, making it possible to improve the energy endurance of the current/voltage non-linear resistor. If the silicon content is less than 0.01 mol %, expressed as SiO2, the benefit of increased strength of the sintered body and improved energy endurance is not obtainable. Furthermore, if the silicon content is more than 1 mol %, expressed as SiO2, the non-linear resistance characteristic is adversely affected.
Sb2O3 has a benefit of forming spinel grains in the sintered body and suppressing growth of ZnO grains. Also, Bi2O3 provides a liquid phase during the sintering process and has a benefit of promoting ZnO grain growth. The resistance of a current/voltage non-linear resistor whose main component is ZnO depends on the number of grain boundaries of the ZnO grains contained in the sintered body, at which a non-linear resistance characteristic is produced, so that the resistance becomes higher as the ZnO grains become smaller. Consequently, in the present invention, the resistance of the current/voltage non-linear resistor can be improved by suppressing ZnO grain growth in the sintered body by making the ratio of Bi2O3 content to Sb2O3 content below 0.3. If an improvement in the resistance of the current/voltage non-linear resistor could be achieved, the number of sheets of current/voltage non-linear resistor laminated in the lightning arrester would be reduced, so that the size of the lightning arrester could be decreased.
Still furthermore, according to the present invention, the grain size distribution of the ZnO grains can be made more uniform by including 0.1 to 1000 ppm of zirconium, yttrium or iron, expressed as ZrO2, Y2O3 or Fe2O3. Consequently, by forming the grain boundaries of the ZnO grains uniformly, the non-linear resistance characteristic that appears at the grain boundaries of the ZnO grains can be improved. Furthermore, since the trace additions of ZrO2, Y2O3 or Fe2O3 are dispersed in the ZnO crystal grains, the strength of the current/voltage non-linear resistor and energy endurance characteristic thereof can be improved. Consequently, even if the energy disposal rate per unit volume is increased, the current/voltage non-linear resistor is fully capable of withstanding this energy, so that the reduction in size of the current/voltage non-linear resistor can be achieved. If the content of zirconium, yttrium or iron expressed as ZrO2, Y2O3 or Fe2O3 is less than 0.1 ppm, the improvement in the non-linear resistance characteristic and the energy endurance characteristic cannot be achieved. Further, on the other hand, if the content of zirconium, yttrium or iron is more than 1000 ppm expressed as ZrO2, Y2O3 or Fe2O3, the non-linear resistance characteristic is adversely affected.
In a further aspect of the present invention, there is provided a current/voltage non-linear resistor comprising a sintered body having a main component of ZnO, an electrode and an insulating material provided for the sintered body, the sintered body having a disc- or ring-shaped structure having a resistance increasing progressively from edge portions of the sintered body towards an interior in the radial direction thereof.
In a preferred example of this aspect, when a voltage of 1.1 times to 1.4 times the voltage at a time of flowing a current of 1 mA is applied and assuming that a current density of each region of the current/voltage non-linear resistor when the voltage is applied is Jv (A/mm2), a gradient per unit length in the radial direction of the current density Jv from the edge portions of the sintered body to the interior in the radial direction thereof is more than xe2x88x920.003 and less than 0. Furthermore, when a voltage of 1.1 times to 1.4 times the voltage at a time of flowing a current of 1 mA is applied, a distribution of the current density Jv (A/mm3) is within xc2x180% in a region of the current/voltage non-linear resistor when the voltage is applied.
According to this aspect, one mode of breakdown of a current/voltage non-linear resistor at a time of absorbing the surge energy includes a thermal (heat) stress breakdown. In the thermal stress breakdown, a heat is generated unevenly because, when Joule heating occurs on the absorption of surge energy by the current/voltage non-linear resistor, the distribution of the electrical resistance within the current/voltage non-linear resistor is not necessarily uniform. This generation of the heat will produce the thermal stress in the current/voltage non-linear resistor, causing breakdown of the current/voltage non-linear resistor. Since cracks produced by the thermal stress occurs from the edges of the current/voltage non-linear resistor, by moderating the thermal stress on the edges of the current/voltage non-linear resistor, the thermal stress breakdown can be suppressed and the surge energy endurance thereby improved.
Furthermore, the temperature distribution, resulting from the heat generation when the surge energy is absorbed by the current/voltage non-linear resistor, is the current distribution when the fixed voltage is applied to the electrodes at both end surfaces in a current/voltage non-linear resistor having disc shape or ring shape.
Consequently, the resistance distribution in the thickness direction of the current/voltage non-linear resistor has no effect on the temperature distribution resulting from the heat generation, and since a resistance distribution in the peripheral direction of the current/voltage non-linear resistor is unlikely to be produced in the manufacturing process, the resistance distribution that does affect thermal stress breakdown, i.e. the temperature distribution resulting from heat generation, is the resistance distribution in the radial direction of the current/voltage non-linear resistor.
The effect of the resistance distribution in the radial direction on the heat stress at the edges of the current/voltage non-linear resistor is component, and the temperature produced by heat generation becomes progressively higher as the edges approach due to the adoption of a resistance distribution in which the resistance progressively increases from the circumferential edges towards the interior. Therefore, compressive thermal stress acts at the edges and, even if a large surge energy is absorbed by the current/voltage non-linear resistor, the generation of cracks due to the heat stress becomes unlikely, so a current/voltage non-linear resistor of excellent energy endurance characteristic can be obtained.
Furthermore, if, on the application of a voltage of 1.1 times to 1.4 times of the voltage when a current of 1 mA is flowing, the gradient per unit length in the radial direction of the current density Jv (A/mm2) from the edges of the sintered body to its interior in the radial direction of the sintered body is made to be more than xe2x88x920.003 (A/mm3) and less than 0 (A/mm3), the current density of each region of the current/voltage non-linear resistor being Jv (A/mm2), the thermal stress at the circumferential edges of the current/voltage non-linear resistor acts in compression, and the breakdown due to the current concentration is unlikely to occur, so the energy endurance characteristic can be improved.
Although, in principle, if the gradient per unit length in the radial direction of the current density Jv (A/mm2) from the edges of the sintered body to its interior in the radial direction of the sintered body is 0 (A/mm3), the temperature distribution at the periphery of the current/voltage non-linear resistor would be uniform, in practice, it is difficult in point of view of the manufacturing process to achieve completely uniform resistance distribution of the element.
Furthermore, if, on the application of a voltage of 1.1 times to 1.4 times of the voltage when a current of 1 mA is flowing, the distribution of the current density Jv is made to be within xc2x180% in all regions of the current/voltage non-linear resistor, the thermal stress generated in the vicinity of the regions of the maximum temperature or regions of the minimum temperature of the heat generation temperature in the interior of the element can be reduced and current concentration in regions of low resistance can be suppressed, thus enabling excellent energy endurance to be achieved.
According to still further aspect of the present invention, there is also provided a sintered body for a current/voltage non-linear resistor having a main component of ZnO, wherein the main component contains, as auxiliary components, Bi, Co, Mn, Sb, Ni and Al, the contents of the auxiliary components being respectively expressed as Bi2O3, Co2O3, MnO, Sb2O3, NiO and Al3+, of Bi2O3: 0.3 to 2 mol %, Co2O3: 0.3 to 1.5 mol %, MnO: 0.4 to 6 mol %, Sb2O3: 0.8 to 7 mol %, NiO: 0.5 to 5 mol % and Al3+: 0.001 to 0.02 mol %; a Bi2O3 crystalline phase in the sintered body including an xcex1-Bi2O3 phase representing at least 80% of the total Bi2O3 phase.
In another aspect, there is also provided a sintered body for a current/voltage non-linear resistor comprising a main component of ZnO, wherein the main component contains, as auxiliary components, Bi, Co, Mn, Sb, Ni, Al and Te, the contents of said auxiliary components being respectively expressed as Bi2O3, Co2O3, MnO, Sb2O3, NiO, Al3+ and TeO2 of Bi2O3: 0.3 to 2 mol %, Co2O3: 0.3 to 1.5 mol %, MnO: 0.4 to 6 mol %, Sb2O3: 0.8 to 7 mol %, NiO: 0.5 to 5 mol %, Al3+: 0.001 to 0.02 mol % and TeO2: 0.01 to 1 mol %; a Bi2O3 crystalline phase in the sintered body including an xcex1-Bi2O3 phase representing no more than 10% of the total Bi2O3 phase.